A flexible printed board is produced in such a manner that, for example, a flexible printed board main body having a circuit of copper foil formed on a polyimide substrate, is sandwiched between coverlay films having an adhesive coated on one side of each coverlay film to form a laminate, and the laminate is sandwiched between press boards and subjected to hot press to press-bond the coverlay films to the flexible printed board main body.
At the time of hot press, a cushion material made of silicone rubber is interposed between the laminate and the press boards in order to absorb unevenness of a reinforcing substrate and the circuit of the flexible printed board main body and to laminate the flexible printed board main body and coverlay films without any space.
Further, in order to prevent the adhesive of the coverlay film from adhering to the cushion material, a release film is located between the cushion material and the coverlay film.
However, since the release film does not adhere to the cushion material, it is necessary to set or change it for every hot press. Therefore, the productivity of the flexible printed board tends to be insufficient, and the cost tends to be high. On the other hand, if the release film is adhered to the cushion material by an adhesive, etc., the release film is strongly adhered to the cushion material by repetitive hot press, whereby the release film cannot easily be removed from the cushion material at the time of changing the release film having unevenness of the flexible printed board main body transferred.
Therefore, a mold releasable cushion material is suggested wherein a releasing property is imparted on the surface of the cushion material (Patent Documents 1 and 2).
However, with respect to the releasable cushion material, unevenness of the flexible printed board main body is transferred to the releasable cushion material by repeating hot press, whereby it is necessary to change the releasable cushion material often. Therefore, the productivity of the flexible printed board tends to be insufficient, and the cost tends to be high.
Patent Document 1: JP-A-2000-052369
Patent Document 2: JP-A-2002-144484